The fabrication of integrated circuits (IC) in the semiconductor industry typically employs plasma discharge, especially a capacitively coupled plasma (CCP) discharge, to create and assist surface chemistry within a processing chamber necessary to remove material from and deposit material onto a substrate. Wafer size continuously increases and after transition from 300 mm has already reached 450 mm in diameter. Therefore, the wafer surface area has increased by 2.25 times, and providing the required plasma density for the etching process becomes a problem.
For a 450-mm wafer, the scaling method that previously has been successfully used does not apply and needs some correction in order to compensate for the increase in RF generator power. This occurs because after exceeding some critical value, further increase of RF power will lead to extra high voltage and to electrical breakdown and arcing inside the plasma processing chamber. This drastically reduces reliability and cost of the system.
However, even though the RF power used for supporting the plasma etching processes is increased, in treating wafers of increased diameters, simple scaling of tools in accordance with Moor's Law can be used to some extent, taking into account similarity in design, technology, etc. One solution to this problem is based on the fact that an increase in frequency improves power coupling of the showerhead (as an electrode) with capacitive discharge. This, in turn, improves power coupling to the plasma. Plasma density also increases since plasma potential is increased. At the same time, sheath screening decreases. High-frequency excitation allows for obtaining much higher ion fluxes (i.e., plasma density) than the classical 13.56-MHz excitation at the same RF power.
However, at frequencies much higher than 13.56, the electromagnetic effects cause some problems that deteriorate the uniformity of plasma density. This nonuniformity may result mainly because of the following three factors: (1) the standing-wave effect that influences RF power deposition at the center of the plasma bulk; (2) an edge effect known as the “telegraph effect,” which is caused by the reflection of RF power from the edges of the showerhead and is characterized by some plasma density perturbation in the vicinity of the edges; and (3) the skin effect, which enhances RF power deposition near the edges of the showerhead and tends to increase the local plasma density in the vicinity of the edges.
All of these factors cause drastic changes in distribution of plasma density and ion flux and eventually result in nonuniformity of etching. In other words, enhancement of uniformity is a key concern for transition to high-frequency excitation.
All current capacitively coupled plasma (CCP) tools are built on the principle of using dual frequency for separately controlling the amount of ions and radical fluxes on one hand, and, on the other hand, distribution of ion energy applied to the wafer. A reactive ion-etching system with dual frequency typically consists of a parallel-plate plasma-etching chamber where a CCP discharge is generated between an upper electrode or cathode and a lower electrode with a wafer. A conductive silicon wafer held by an electrostatic chuck is surrounded by a silicon focus ring and a dielectric outer ring. The wafer, the electrostatic chuck, and the focusing ring are combined into a wafer system. Both electrodes (cathode and wafer) are joined through matching networks to separate RF generators with frequency ratios from (10:1) to (10:5), where a higher-frequency generator is connected to the upper electrode (cooler plate), and another generator that operates at a lower frequency is connected to the wafer system through a blocking capacitor.
These two sources must be decoupled since the upper electrode operates at high frequency, e.g., 150 MHz, and the low electrode operates at low frequency, e.g., 13.56 MHz. FIG. 1 is a schematic transverse sectional view of a showerhead/cooler plate assembly that consists of a showerhead 20 and a gas-feeding cooler plate (hereinafter referred to as “cooler plate”) 22, which when assembled is pressed against the showerhead 20. Due to a recess 24 machined on the surface of the cooling plate 22, a cavity is formed between the mating faces of the showerhead 20 and the cooler plate 22. In FIG. 1, reference numeral 26 designates a chuck that holds an object to be treated in a processing chamber (not shown), e.g., a semiconductor wafer W.
FIG. 2 is a three-dimensional view of the cooler 22 on the side opposite to the shower head 20 and facing the manifold head (not shown in FIG. 2) and contains a manifold pattern of radial grooves 22-1, 22-2, . . . 22-n and concentric grooves 22′-1, 22′-2, . . . 22′-m. 
FIG. 3 shows a view of the cooler plate 22 on the side 30 that faces the showerhead 20. The structure shown in FIGS. 1, 2, and 3 relates to a conventional plasma-treatment apparatus designed to treat 300 mm wafers. To provide uniform etching of the wafers with an outside diameter of 300 mm, the showerhead 22 should have a diameter of ˜380 mm. Furthermore, the showerhead 22 should have a pattern of ˜900 mm uniformly distributed through gas passages 30a, 30b, . . . 30k (FIGS. 1 and 3) having diameters ranging from 1.0 to 1.5 mm.
As mentioned above, the scaling of tools in accordance with Moor's Law still applies to the transition from 300-mm wafers to 450-mm wafers with preservation of the same gas density per unit area but with transition to a showerhead that is drastically increased in size and becoming commensurable with the wavelength of RF power. In this case, however, for uniformity of plasma, the outside diameter of the showerhead should be increased as well in order to exceed diameters of the wafer and of the focusing ring. If the diameter of a showerhead is ˜600 mm, the cooler plate 22 should have an outside diameter of 700 mm or more. The number of gas holes will grow up to 2,000 or more. Similar to the cooler plate used for processing 300-mm wafers in the Etching System TEL 300 of Tokyo Electron Limited (Japan), a new cooler plate 22a may comprise a perforated disk made from aluminum. The shallow radial grooves 22-1, 22-2, . . . 22-n and concentric grooves 22′-1, 22T-2, . . . 22′-m formed on the side 22a of the cooler plate 22 serve to uniformly distribute the processing gas in radial and concentric directions before injection into the gas passages 30a, 30b, . . . 30k (FIG. 3) that are formed on the bottom of these grooves and pass through the entire thickness of the cooler plate 22, terminating on the side 30 (FIG. 3) that faces the showerhead 20 (FIG. 1). As mentioned above, these gas passages usually comprise holes with diameters of 1.0 mm to 1.5 mm. If the number of gas passages in the conventional cooler plate is ˜900, an increase in wafer diameter from 300 mm to 450 mm requires an increase in the number of gas passages by a factor of 2.75 to 3.
The recess 24 (FIG. 1) forms a gas reservoir for maintaining processing gas at a predetermined pressure that is controlled by gas flow controllers and valves (not shown). The pressure in the manifold determines the etching process in the processing chamber The cooler plate 22 also has an array of holes 36a, 36b, . . . 36f for screws that attach the cooler plate 22 to the head of the showerhead and another array of holes 34a, 34b, . . . 34f for screws that attach and press the cooler plate 22 to the manifold head of the entire upper electrode system. The manifold head (not shown) is insulated from the cooler plate 22 and is connected to an RF generator (not shown) through a matching network (not shown) that supplies RF power to the showerhead 20 (FIG. 1) through the cooler plate 22.
Because of the recess in the central part of the cooling plate that faces the showerhead, the RF power attained from the manifold head is uniformly distributed over the cooler plate 22 and is transferred to the showerhead 20 only through the peripheral part of the showerhead where both parts have ohmic contact.
In transition from wafers of 300 mm to wafers of 450 mm, the showerhead 20 (FIG. 1), which in the wafer processing apparatus functions as a cathode, also can be scaled to an increased diameter. The showerhead 20 comprises a perforated disk, the outside diameter of which will be increased when the 450-mm wafers increase in size from 380 mm to 600 mm. As usual, the showerhead 20 has a plurality of gas holes 38a, 38b, . . . 38p (FIG. 1) that supply processing gas from the gas pressure reservoir 24 (FIG. 1) to the processing chamber. Bulk plasma is formed between the front side 20a of the showerhead 20 and the wafer W. However, the law of scaling will not be properly applied during transition to the 450-mm wafers; also, selection of the depth in the gas pressure chamber 24 (FIG. 1) becomes not so obvious and trivial as for the 300-mm wafers. More specifically, exceeding the threshold gap depth leads to the same instability that occurs when frequency exceeds rated value. Theoretically, all RF power is supposed to be delivered from the cooler plate 22 to the showerhead 20 through perfect ohmic contact at the periphery of both contacting parts, but in reality the force that presses the cooler plate 22 to the showerhead 20 by tightening the bolts 36a and 36b should be limited in order to prevent crushing of the very fragile silicon showerhead 20. Therefore, contact between the showerhead and the cooler plate must be taken into account. Furthermore, the polished contact surfaces at the periphery cannot be ideally even and may be contaminated. Therefore, in an actual application we should expect that some RF power can be lost and some gas may leak through the gaps. In order to stabilize the pressure in the gas pressure reservoir 24 (FIG. 1) of the cooler plate 22 and in the working space between the showerhead and the chuck, the diameter of the passages in the cooler plate should be selected with a predetermined ratio to the diameter of the showerhead gas holes. In other words, the showerhead should provide predetermined gas permeability that is represented by the total or regional gas flow through the gas holes and that can be expressed in terms of volumetric flow rate measured in conventional units, such as standard cubic centimeters per minute (sccm). In conventional showerheads, this permeability is uniform. Gas holes, which are drilled in the showerhead, are typically round holes, which may have diameters from 0.5 mm to 1.0 mm. The total number of gas holes is in the hundreds, e.g., up to 1,000 or more. Gas permeability in these passages can be adjusted by decreasing or increasing the number or the diameter of the holes, or both.
In order to provide uniform injection of processing gas from the gas pressure reservoir 24 to the plasma processing space, the gas-feeding passages 30a, 30b, . . . 30k (FIG. 1) of the cooler plate 22 are generally aligned with the gas distribution holes 38a, 38b, . . . 38p of the showerhead 20 (FIGS. 1 and 2), and their number is equal. Furthermore, as mentioned above, the passages 30a, 30b, . . . 30k of the cooler plate 22 and the gas holes 38a, 38b, . . . 38p of the showerhead 20 are arranged in a pattern of concentric circles in order to provide concentric and radial uniformity of gas distribution according to the geometry of the wafer W. In order to provide uniformity of plasma etching, both the showerhead 20 and the cooler plate 22 are round in shape, and their diameters are approximately 1.5 times greater than the diameter of the wafer to be treated.
The showerhead is made from a conductive, high-purity material such as a single-crystal silicon, polycrystalline silicon, or silicon carbide. A chemically reactive gas such as CF4, CHF3, CClF3, SF6 or mixture thereof with O2, N2, He or Ar is introduced into the plasma processing space and is maintained at a pressure that is typically in the range of milliTorrs.
It was found that the diameter of 0.5 mm for the gas holes is optimal for providing an indispensable gas flow rate and simultaneously reducing the influence of pressure in the gas reservoir on vacuum conditions in the processing chamber. On the other hand, since the diameters of the gas holes are small, in spite of the fact that a large amount of the holes is exposed to the processing side of the showerhead, the surface of the showerhead side remains relatively smooth, and uniformity of the plasma sheath, which is very sensitive to the surface condition of the processing side of the showerhead, is not deteriorated either by the matrix of holes or by surface roughness. As a result, due to the drop in pressure developed in the gas pressure reservoir 24 (FIG. 1) and in the processing chamber, the turbulent flow of processing gas through the gas-feed passages 30a, 30b, . . . 30k of the cooler plate 22 is redistributed to a more static state and is converted to a laminar flow delivered to the plasma processing chamber at a relatively uniform rate.
It is generally required that passage dimensions and spacing be strictly controlled between gas holes of the showerhead used in the gas delivery system in order to provide uniform gas distribution on a particular surface area of the showerhead. The plasma that is used for etching semiconductor wafers is very corrosive. Naturally, a showerhead that is used in such a process is subject to the effect of very chemically corrosive gasses, especially when etching the wafers with highly reactive radicals, and therefore the showerhead is subject to deterioration by chemical corrosion. Moreover, heavy ions generated during plasma discharge bombard the sharp edges at the exits of the gas holes where electrical strength is very high. As a result, such exits are converted into craters, and this violates the etching process. Therefore, a showerhead is a consumable part that is supposed to be periodically replaced.
However, the showerhead is a very expensive item. Therefore, instead of replacing the entire showerhead, the eroded part on the processing side can be removed by re-polishing. After re-polishing, the processing side of the showerhead can serve two more additional terms. An electrical and mechanical contact between the cooler plate and the showerhead is provided through the periphery portions of both parts. In other words, RF voltage is applied to the periphery of the showerhead backside through ohmic contact. The rest of the showerhead backside surface does not have such contact because of the recess 24 (FIG. 1) formed in the cooler plate 22. Therefore, the central part of the showerhead 20 cannot transfer RF power. Thus, RF power is supposed to propagate from the edges in the radial inward direction to the center of the showerhead 20 through a thin layer on the processing side of the silicon-made showerhead. Generally, the surface resistance of silicon doped with boron is ˜75 ohm·cm. Therefore, at a conventional frequency of 13.56 MHz, surface impedance allows the launching of electromagnetic waves into the plasma from the total surface of the showerhead rather than from the edges where the showerhead is electrically connected to the cooler plate.
For showerheads of relatively small diameters, e.g., those intended for processing 300-mm wafers and operating at low frequencies of RF power, e.g., 13.56 MHz, the transfer of power from the cooler plate 22 to the showerhead 20 through the edges, i.e., through the areas of contact between the cooler plate 22 and the showerhead 20, does not drastically affect uniformity of the plasma generated under the working face of the showerhead in the processing space of the plasma processing chamber. This occurs because the wavelength of RF power is much larger than the diameter of the showerhead, and in this case RF power uniformly propagates from the edges to the center and is transferred to the plasma discharge from the total surface of the showerhead into the conductive bulk plasma.
This discharge ionizes and dissociates the reactive gas that forms plasma, thus generating ions and chemically active radical particles. The ions strike the surface of the wafer to be etched by chemical interaction and momentum transfer. Because ion flow is predominantly normal to the surface of the wafer, the process produces the well-defined vertically etched sidewalls. The highly reactive radicals are not charged and have the ability to penetrate even the narrow and deep trenches in the wafer to provide etching there. An ion bombardment of energy is influenced by excitation in the plasma sheath adjacent to the wafer because low-frequency voltage is applied to the bottom of the chuck (lower electrode).
Thus, the level of power introduced into the system at low frequency provides control of coordinates and angular distribution of ion energy across the surface of the wafer. When high-frequency power is applied to the showerhead (upper electrode) from the cooler plate through the zone of contact with the showerhead, plasma density is controlled by high currents that are displaced more significantly toward the aforementioned zone of contact and increase the ohmic power transferred to the plasma and cause heating of the plasma sheath. In other words, under the conditions described above, RF power of high frequency is responsible for generation of ions and radicals. Because the system operates at dual frequencies, it becomes possible to adjust plasma density and ion bombardment energy separately.
RF power is supplied from an RF power supply unit through a matching network (not shown) and the cooler plate 22 to the backside 20b of the showerhead 20 (FIG. 1), specifically through the peripheral area of the latter (as mentioned above, the central part of the cooler plate 22 is occupied by the recess 24). The power is transmitted through the showerhead surface 20a to the plasma. When frequency increases, electrical field intensity at the central part of the showerhead working surface becomes higher than the electrical field intensity at the peripheral portion of the showerhead 20. Therefore, the density of generated plasma is higher at the central part of the processing space than at the peripheral part of this space. As a result, the uniformity of plasma density further deteriorates, which results in poor planar uniformity and charge-up damage to the plasma etching.
With the increase of showerhead size and much higher frequency requested to support the optimal level of plasma density for maintaining uniformity in dual frequency, capacitively coupled plasma-etching systems become more complicated because of the electromagnetic and finite wavelength effects that deteriorate this uniformity. The main source of plasma nonuniformity at an ultra-high frequency is the so-called “standing-wave effect.”
At extra-high frequencies, RF power applied to the rear peripheral side of the showerhead is concentrated mostly at the edges and does not propagate to the center through the surface layer of the showerhead for entering the plasma. At these frequencies, impedance of a plasma sheath becomes less than the impedance on the surface of the showerhead.
Therefore, it is preferable to apply RF power directly to the plasma rather than first to the processing surface and then to the plasma. Thus, RF power enters directly into the plasma and more specifically into the plasma sheath in the vicinity of these edges. Once entering the plasma, the electrical field does not significantly penetrate the plasma bulk but appears to be wave-guided in the sheath.
As RF frequency increases, the plasma-effective wavelength decreases, and therefore the uniformity of the electrical field worsens. At 150 MHz, the size of the showerhead becomes comparable to or less than the wavelength of RF power propagated in the plasma. As mentioned above, at 150 MHz, the showerhead radius is larger than the quarter wavelength, and in this case the phase change of RF power from the edge to the center of the showerhead also becomes greater than the quarter wavelength. Moreover, transition to sufficiently high frequencies shortens the wavelength and leads to some constructive and destructive interferences and skin effects. Because of the constructive interference of counter-propagating waves from the opposite sides of the showerhead, the amplitude of the electrical field in the sheath increases at the showerhead center. This causes nonuniform distribution of plasma density, with plasma density higher in the center than at the edges. Therefore, depending on the frequency, the finite wavelength produces nonuniformities that are already problematic for 300-mm showerheads, and will become more critical for showerheads used in apparatuses treating wafers of 450 mm. Thus, at the frequency of 150 MHz, the transition occurs from a traveling wave to a standing wave. This phenomenon causes interference of the aforementioned wave with the counter-propagating waves reflected from the rear sides of the showerhead.
The electrical field launched by the RF power and plasma current introduced into the plasma becomes highly nonuniform with the amplitude of the electrical field in the plasma sheath increased at the center of the showerhead (electrode). Several simulations made by different authors [see L. Sansonnens and J. Schmitt, Appl. Phys. Lett. 82, 182 (2003)] show that the electrical field is maximal at the center of the discharge and decays toward the edges, thus following the Bessel function. Such changes in RF power distribution result in nonuniform RF power deposition into plasma. As a result, the wafer treatment processes such as etching or deposition become nonuniform as well.
The local deposition of RF power in plasma that occurs near the edges of the showerhead and leads to increased local plasma density at the edges is referred to as the “skin effect.”
For a CCP reactor with the geometry described above, argon plasma at the RF power frequency of 150 MHz (450 W) is sustained at a gas pressure of 50 mTorr. Under these conditions, ion flux density along the showerhead radius has the following values: the plateau around the center and to the radius of 50 mm of the showerhead has an ion flux density equal to I=4.75×1015 cm−2 s−1; the lower plateau in the area from the radius of 150 mm to the edge that has a total radius of 240 mm has an ion flux density I=1.75×1015 cm−2 s−1; and the linear downfall branch has an ion flux density I decreasing with radius R and expressed by the following formula:I=4.75×1015 cm−2 s−1(1-bR),where b is in the area with a radius from 50 mm to 150 mm at ˜2.75×10−2.
FIG. 4 shows distribution of ion flow at frequencies of 13.56 MHz, 60 MHz, and 81.38 MHz for processes treating a 200-mm wafer. It can be seen that the ion flux drops almost by a factor of three at the edges as compared to the central area of the wafer. When at a high frequency the size of a wafer is doubled and the size of the cathode approaches the wavelength of the RF field, we can expect other effects that contribute to deterioration of scaling. Thus far, numerous suggestions have been proposed to provide uniformity of etching.
The article, “450-mm Dual Frequency Capacitively Coupled Plasma Sources: Conventional, Graded, and Segmented Electrodes” published by Yang Yang and Mark J. Kushner in the Journal of Applied Physics 108, 113306 (2010), presents results from a two-dimensional computational investigation of Ar plasma properties in a 450-mm DF-CCP reactor incorporating a full-wave solution of Maxwell's equations. The authors taught that finite wavelength effects followed by increase in frequency lead to collapse of electron density even in the plasma reactor for 300-mm wafer processing. The authors showed that in the center, in the intermediate portion, and at the edges of the electrode, the ion flux incident onto the wafer, ion energy, and angular distributions (IEADs) are different. It was suggested to solve the problem of plasma nonuniformity in the plasma-processing cavity by redistribution of RF power by providing graded conductivity electrodes with a multilayer of dielectrics. This dielectric coating changes the surface impedance that becomes less than the impedance of the plasma sheath in order to attract RF power and to return movement of RF power to the processing side of the showerhead. Thus, in order to enhance plasma uniformity, RF power is again redistributed on the processing surface of the showerhead in a specific order from the edges to the center of the showerhead. It is also stated that “segmentation of the HF electrode also improves plasma uniformity by making the electrical distance between the feeds and the sheath edges as uniform as possible.”
In the above structure, the finite wavelength effect is lessened by decreasing the conductivity of the plasma-contacting surface of the electrode from the edge to center of the electrode by using a coating of dielectric material with a variable density. In this case, the RF wave can readily propagate through the surface of the showerhead dielectrics rather than through the plasma sheath, especially at the center of the showerhead having the lowest conductivity. Thus, the peak of electron density at the center is diffused and uniformity becomes smoother.
A drawback of this method is erosion and sputtering of the dielectric layer during the plasma process. As a result, plasma density distribution changes from process to process. Another drawback is contamination of the wafer by the sputtered material.
Other different methods to suppress the effect of electromagnetic waves were proposed. For example, Japanese Unexamined Patent Application Publication (KOKAI) No. 2000-323456 published on 11.24.2000, inventor A. Koshiishi, describes a plasma-processing device wherein the showerhead consists of two parts and the central part of the showerhead is made from a high resistivity material for consuming more RF power because of Joule heating. As a consequence, the electrical field intensity is reduced to a greater extent in the central part than at the peripheral portion of the showerhead. This effect is used for leveling the distribution of plasma density. However, the high resistivity part of the showerhead consumes too much RF power during Joule heating, and this reduces the efficiency of the device.
Another method to improve uniformity of ion flux onto a wafer is to use the so-called slot antenna. U.S. Pat. No. 8,080,107 issued to W. Kennedy, D. Jacob on Dec. 20, 2011 describes a showerhead that consists of two to six separate segments arranged in a ring configuration, such as segments of single-crystal silicon. But Yang Yang and Mark J. Kushner [see Journal of Applied Physics 108, 113306 (2010] suggested splitting the RF power and the power at these segments at different phases. At the segments, the phases of RF voltage alternate with 180°. The in-phase excitation retains the character of a surface wave propagating along the sheath and thus higher-density plasma is formed in the center. However, out-of-phase excitation shifts the maximum plasma density from the center to the mid-radius. This middle-peaked plasma density may lead to excitation of a higher order of waveguide mode in the chamber. As a result, it becomes more difficult to adjust the uniformity of plasma density.
A drawback of this method is a complicated, real-time control of plasma uniformity that includes tuning the phases by oscillating the phases of the segments or the phase swapping to shift the pick of RF power distribution from the center to the middle. The metal ceramic neighboring at the processing surface of the showerhead deteriorates the plasma sheath, and the resulting sputtering and erosion contaminate the product.
Sansonnens and Schmitt [see L. Sansonnens and J. Schmitt, Appl. Phys. Lett. 82, 182 (2003)] proposed to solve the problem of plasma-density nonuniformity by fabricating a Gaussian-shaped surface profile on electrodes covered with a thin dielectric plate to confine the plasma in a constant inter-electrode gap. In this proposal, a dielectric lens should have a Gaussian shape in order to receive uniform voltage across discharge and thus suppress the standing-wave effect. However, manufacture of the showerhead with an accurate and smooth curvilinear surface is an extremely complicated, inefficient, and expensive procedure.
There exist many other methods and devices for improving uniformity of plasma density distribution in the plasma-processing cavity of a CCP processing apparatus. However, in the majority of cases, these methods and apparatuses are aimed at solving the above-stated problem by managing the distribution of RF power.
It is understood in this regard that in transition to 450-mm wafer etching systems, the above methods and constructions are less efficient than methods based on controlling gas distribution. There exists a large number of gas holes of the same geometry in the showerhead for introducing processing gas from the gas reservoir to the plasma-processing chamber. The diameter of the gas holes is approximately 0.5 mm. The separation between the neighboring gas holes may vary from 5 mm to a greater distance. The gas flow through each hole is the same. However, changing the geometry at the exits of the holes on the processing side of the showerhead is not recommended (see, for example, U.S. Pat. No. 6,333,601 issued to S. Wickramanayaka on Dec. 25, 2001, “Planar gas introducing unit of a CCP reactor”). It is taught that with the increase in the gas-hole diameter to a value greater than 0.5 mm, the processing plasma will penetrate deeply into the hole and will increase erosion rate at the hole exit.
It is known that positive ions of plasma accelerate toward a showerhead surface and bombard this surface. These ions gain high energy, especially in vicinity of the sharp edges of the gas holes where density of the electrical field is high; thus, the bombardment of ions on the surface causes sputtering of this surface. According to this theory, the sputtering damage is higher at the exit of the gas holes since there is higher gas density and higher plasma density at these points. This process causes an erosion of the gas hole compared to the other areas of the showerhead, resulting in an enlargement in the diameter of the gas holes. With increased gas-hole diameter as a result of erosion, plasma tends to confine in the vicinity of the exit of these holes by multiple reflections of electrons from the walls of the gas holes. Accordingly, the erosion rate in the gas hole accelerates with the increase of plasma density. This process leads to the tapering of gas holes, and eventually the total processing surface of the showerhead should be re-polished.
In order to avoid degradation at the gas-hole exits, all gas distribution enhancing means for uniform plasma etching should be provided at the back side of the showerhead. A conventional method (by LAM® Research Corporation) is to divide the gas pressure reservoir into several separated zones. For example, as disclosed in US Patent Application Publication 20100252197 (inventors Babak Kadkhodayan and Anthony De La Llera; “Showerhead electrode with centering feature,” published Oct. 7, 2010), the gas pressure reservoir is divided into two zones, where about 60% of the gas holes are in an inner zone and preferably about 40% of the gas holes in an outer zone. These zones are separated from each other by a gas sealing, such as an O-ring. Thus, the inner and outer zones above the wafer that undergo plasma etching can be fed with gas at different flow rates to optimize etch uniformity. However, during wafer processing, such as plasma etching, the showerhead and cooler plate are heated, and the difference in their thermal expansions places high loads on the O-rings. Besides a high temperature, the O-ring is also exposed to a highly corrosive etching gas. As a result, the O-ring material degrades, thus contaminating the etching process. Because processing gas leaks through the deteriorated O-ring, the pressure in each zone becomes out of control, and the etching process is deteriorated.
US Patent Application Publication 20060180275 A1 (Inventor Robert Stagger; published on Aug. 17, 2006) discloses a method of manufacturing gas distribution members for plasma-processing apparatuses. The method is based on combined control of gas distribution over different plasma zones with different plasma density to upgrade uniformity.
U.S. Pat. No. 8,043,430, “Methods and apparatuses for controlling gas flow conductance in a capacitively coupled plasma processing chamber,” issued on Oct. 10, 2011 to R. Dhindsa, et al, discloses an apparatus for controlling flow conductance of plasma in plasma processing. The apparatus includes a ground ring that concentrically surrounds the lower electrode and has a set of slots formed therein, and a mechanism for controlling gas flow through these slots. However, the apparatus has a complicated structure, strict tolerances, and must be very carefully designed just for a single-process protocol. Another factor that causes nonuniformity of plasma treatment is deterioration of gas distribution channels that supply the processing gas from gas accumulation to plasma treatment through the showerhead.
However, since a showerhead surface is under high-voltage RF, the plasma treatment procedure is jeopardized by the highly destructive electrical processes. The plasma treatment chamber is supposed to work under heavy-duty conditions with intensive depreciation of materials. More specifically, instabilities in the RF circuit that are caused by many reasons may lead to generation of very high frequency in RF power. Impedance of the gas reservoir is drastically reduced, and the RF power with extra-high frequency prefers to propagate from the cooler plate to the showerhead through gaseous gaps and through the gas reservoir rather than through the ohmic contact surface on the periphery. A valuable part of energy is diverted by this capacitor-type resistance from the ohmic resistance at the periphery. The gas in the reservoir is also susceptible to electrical breakdown. A corona discharge and arcing also occur in this area. Further, the above-described abnormal conditions lead to a phenomenon that is known as a hollow cathode discharge, which occurs on the developed surfaces at the entrances to the aforementioned gas passages and penetrates inside the gas holes. This leads to the loss of power and distortion of the passage geometry, and, hence, to instability in technological parameters of the process. Under the effect of the hollow cathode discharge, in the worst case the gas flow becomes totally ionized or becomes the carrier of the charged particles that are introduced into the processing plasma and can be converted into miniature arcs. The arcing overheats the inner part of the holes of the exit areas and changes the structure of silicon. This leads to drastic lowering of the resistance of silicon to sputtering and etching. The heavy ions that are generated in the processing plasma bombard and sputter the overheated edges of the gas channels. They even develop craters that may reach 3 mm in depth or more.
Radicals penetrate into the passage deeper than ions and expand the initial diameter of the channel two to three or more times. In other words, it can be assumed that the aforementioned degradation of surface of the showerhead that faces a plasma discharge in the etching process can be explained by interaction of the charged particles and radicals on the plasma-surface boundary of the showerhead.
A source of deterioration of the showerhead surface is ionization of a gas flowing through each gas hole that is capable under some conditions to generate its own plasma discharge. Such a discharge, in turn, generates ions that bombard the passage wall. Moreover, for mismatching of impedances of the RF power supply with the processing system, the ionized gas flow can be easily converted into an arc. Such a mismatching may be caused by variations in chamber pressure, RF power, etc. In this case, a high-temperature torch that occurs at the exit of a gas hole causes thermal erosion on the surface of the showerhead and funnels the gas holes by creating a nozzle effect in the vicinity of the border between the exit of the passage and bulk plasma. Consequences of this effect are aerodynamic expansion, turbulence of gas ejected into the chamber, deterioration of uniformity of a plasma density, and contamination of the processing chamber, especially, the periphery of the showerhead by deposition of the product of erosion.
On the other hand, a corona discharge that causes arcing occurs in the gap on the gas input edges of the gas supply channels, i.e., on the side of the showerhead that faces the cooler plate. This arcing leads to destruction of the showerhead and, hence, to nonuniform distribution of processing gas in the plasma cavity, as well as to contamination of processing gas and of the product with particles of the showerhead material.
It should be further noted that the deterioration described above is not uniform and has a different degree in different areas of the showerhead. For example, gas-directing passages located closer to the periphery of the showerhead deteriorate faster and at a greater degree than in the center of the showerhead. This leads to shortening of the channel lengths in the peripheral part of the showerhead, which leads to decrease in gas pressure in the space in the peripheral areas of the processing chamber. This, in turn, leads to the nonuniformity of plasma.